In April 1998 the transient source XTE J1808-369 was observed by RXTE,
and found by Frank Marshall to be positionally coincident with
SAX J1808.4-3658 which had been observed during a previous outburst
by SAX in September 1996.
Analysis of the public RXTE data by Rudy Wijnands and
Michiel van der Klis
revealed a sharp peak in the power spectrum at
401 cycles per second.
The extreme stability of this frequency over time led to
the discovery that this object is a millisecond
pulsar, the first such object found to be
accreting matter from a companion star.

Additional analysis by Deepto Chakrabarty and Ed Morgan
uncovered variations in the times at which the
signals from the pulsar arrival at the Earth.
From delay times of individual pulses,
these workers
found
that the pulsar lies in a binary with a 2.01
hour orbital period.
The finite size of the
orbit of the pulsar about its
companion means that the pulses sometimes arrive
earlier than expected, and sometimes later than expected.
A plot of the observed-minus-expected arrival times
versus time has a perfectly sinusoidal shape,
indicating that the pulsar's orbit is circular.
The mass-losing companion star is inferred to have a low
mass - about 0.1 solar masses.
There are indications that the companion
is being cannibalized by X-ray heating
from the pulsar. The heating
produces a wind of outflowing gas from the
star, which may lead to its complete evaporation in
another 10 to 100 million years.

Millisecond pulsars are thought to be "spun-up" to high frequencies
by the accretion of matter, a process which takes several hundred
million to billions of years.
Therefore millisecond pulsars are very old,
in contrast to pulsars like the one in the Crab nebula
which spins 30 times per second
and is only about 1000 years old. Millisecond pulsars have
been seen throughout the galaxy at radio wavelengths,
but always in systems where the mass losing companion has
stopped transferring material to the pulsar; the presence
of accreting gas in the vicinity of the pulsar tends to block
the radio signal from the pulsar. SAX J1808.4-3658
is the first
such object for which a persistent
millisecond signal has been seen X-rays.
Spinning once every 2.5 milliseconds, a point
on the equator of this pulsar
is moving at about 10 percent the speed of light.

Figure 1: Power Spectrum of RXTE light curve of SAX J1808.4-3658 from 1998 April 11.

A strong peak at the pulsar spin frequency is
evident in the power spectrum (Wijnands & van der Klis 1998, Nature, 394, 344).

Figure 2: Pulse arrival time delays due to the ~2.01 hr orbit of SAX J1808.4-3658.

The pulses from the pulsar arrive either earlier than expected or
later depending on where in its orbit the pulsar is. The 126
millisecond amplitude of the pulse delay curve
represents the projected
light travel time across the pulsar's orbit -
corresponding to about 38,000 kilometers.
The binary orbital period
is 7249.1 sec (Chakrabarty & Morgan 1998, Nature, 394, 346).